Thermosetting resins such as phenolic resin, melamine resin, epoxy resin, unsaturated polyester resin and bismaleimide resin are excellent, on the basis of the property referred to as thermosetting property, in the properties such as water resistance, chemical resistance, heat resistance, mechanical strength and reliability, and hence have hitherto been used in wide industrial fields.
However, there are, for example, the following disadvantages: phenolic resin and melamine resin generate volatile by-products at the time of curing, epoxy resin and unsaturated polyester resin are inferior in flame retardancy, and bismaleimide resin is extremely expensive.
For the purpose of overcoming these disadvantages, there have been studied benzoxazine compounds that undergo the ring-opening polymerization reaction of the benzoxazine rings, and that are thermally cured without evolving problematic volatile matter. Such benzoxazine compounds are disclosed, for example, in Patent Document 1.
As excellent benzoxazine compounds that are excellent in dimensional stability, low water absorption, low dielectric constant and heat resistance, and that do not evolve by-produced gases at the time of molding, the benzoxazine compounds represented by the following formula (1) and the following formula (2) are known (Non-Patent Documents 1 and 2):

The resins obtained by the ring-opening polymerization of the benzoxazine rings of the benzoxazine compounds represented by the formulas (1) and (2) do not involve the evolution of volatile components at the time of thermosetting, and are also excellent in flame retardancy and mechanical properties.
Patent Document 2 discloses a thermosetting benzoxazine resin exhibiting flexibility and toughness, and being suitable for electronic parts and the like. However, the technique involved therein allows OH groups to be present in the skeleton of the thermosetting benzoxazine resin and hence the thermosetting benzoxazine resin is disadvantageous from the viewpoints of hygroscopicity and electrical properties.
Furthermore, Patent Document 3 discloses a curable resin having a dihydrobenzoxazine ring structure in the main chain thereof and being excellent in heat resistance and mechanical properties.
However, the curable resin includes highly polar groups such as a sulfone group and hence is thought to be probably disadvantageous from the viewpoint of dielectric properties.
Also, Non-Patent Document 3 and Patent Document 4 disclose a thermosetting resin having benzoxazine rings in the main chain of the resin. However, Non-Patent Document 3 discloses only the compound without description of the evaluation of the properties. Additionally, Patent Document 4 discloses neither guidelines for improving the heat resistance and for imparting the flexibility nor specific compounds.
Non-Patent Document 4 discloses a decomposition mechanism of a cured article of a benzoxazine compound. The aniline and the monofunctional cresol disclosed in Non-Patent Document 4 have volatility at low temperatures.
Furthermore, Patent Document 5 discloses a method for producing a dihydrobenzoxazine compound for which both of a primary diamine compound and a primary monoamine compound are essential as amines.
Patent Document 1: Japanese Patent Laid-Open No. 49-47378
Patent Document 2: Japanese Patent Laid-Open No. 2005-239827
Patent Document 3: Japanese Patent Laid-Open No. 2003-64180
Patent Document 4: Japanese Patent Laid-Open No. 2002-338648
Patent Document 5: Japanese Patent No. 3550814
Non-Patent Document 1: Homepage of Konishi Chemical Ind. Co., Ltd. (retrieved on Nov. 24, 2005), on internet <URL:http://www.konishi-chem.co.jp/cgi-data/jp/pdf/pdf#2.pdf>
Non-Patent Document 2: Homepage of Konishi Chemical Ind. Co., Ltd. (retrieved on Sep. 18, 2007), on internet <URL:http://www.konishi-chem.co.jp/technology/oxazin.html>
Non-Patent Document 3: “Benzoxazine Monomers and Polymers: New Phenolic Resins by Ring-Opening Polymerization,” J. P. Liu and H. Ishida, “The Polymeric Materials Encyclopedia,” J. C. Salamone, Ed., CRC. Press, Florida (1996) pp. 484-494
Non-Patent Document 4: H. Y. Low and H. Ishida, Polymer, 40, 4365 (1999)